The present invention relates to the radiography art. It finds particular application in conjunction with computerized tomographic (CT) scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention may also find application in conjunction with other radiation treatment apparatus and imaging apparatus.
Heretofore, tomographic scanners have commonly included a floor-mounted frame assembly which remains stationary during a scan. An x-ray tube is mounted to a rotatable frame assembly which rotates around a patient receiving examination region during the scan. Radiation from the x-ray tube traverses the patient receiving region and impinges upon an array of radiation detectors. From the radiation data sampled by the detectors and the position of the x-ray tube during each sampling, a tomographic image of one or more slices through the patient is reconstructed.
An x-ray tube generates x-rays by directing a high energy electron bean against a tungsten target. One of the persistent problems in CT scanners and other radiographic apparatus is dissipating the waste heat created while generating x-rays. In higher powered x-ray tubes, the anode rotates so that the high energy electron beam only dwells a fraction of a second at a time on any point on the anode. The x-ray tube is jacketed with a lead lined housing. A cooling oil is circulated between a glass vacuum envelope of the x-ray tube and the lead-lined housing to remove waste heat.
In some scanners, the x-ray tube rotates in one direction during a scan and returns in the other direction for the next scan. Such scanners are normally limited to about 360.degree. of rotation. The single rotation enables the hot cooling oil to be conveyed from the rotating frame by flexible hoses to a non-rotating heat exchanger. Accommodating the cooling oil-carrying hoses is a space consumptive handling problem. Limiting a scanner to about 360.degree. of rotation makes it unable to perform many common diagnostic procedures.
In other CT scanners, the cooling oil is circulated to a radiator or other air-oil heat exchanger that is mounted on the rotating frame portion. This alleviates the hose handling problems and enables the x-ray tube to rotate a plurality of times, e.g., a continuous rotate scanner. However, accommodating the size and weight of the heat exchanger in the tight space constraints of the rotating frame is difficult. As the x-ray tube and rotating frame portion rotate, air passes through the heat exchanger cooling the oil. Limited space on the rotating gantry limits the surface area of the radiator, limiting cooling. In other CT scanners, hot oil or other hot fluid is conveyed to a fluid slip ring. The fluid slip ring is an annular structure that surrounds the patient bore. One part of the slip ring rotates with the rotating gantry and the other part is connected to the stationary gantry. An annular fluid passage is defined between the rotating and stationary slip ring halves. In one prior art design, the hot fluid circulated to a radiator immersed in the fluid in the slip ring. In another design, the hot fluid emptied into the fluid slip ring. In both designs, hot fluid from the slip ring was conveyed to a chiller. Surrounding the patient with a hot fluid carrying slip ring raises serious patient safety concerns. Leaking or failure of the slip ring seals could cause serious burns to the patient.
One of the limiting factors on the speed of a CT scan is the amount of x-rays produced by the x-ray tube. The tube must irradiate each detector for a sufficient duration that each detector receives the minimum total flux needed to reconstruct a good contrast image. Lower power tubes require the tube to dwell or focus longer on each detector. Larger, more powerful x-ray tubes supply the minimum flux more quickly, allowing the speed of x-ray tube rotation to be increased, hence the scan time decreased. However, as the x-ray tubes become more powerful, more heat is generated. More heat is also generated in continuous rotate scanners in which the tube remains "on" during several consecutive rotations for multi-slice imaging.
Larger x-ray tubes, such as seven inch anode x-ray tubes, generate so much heat that the prior art heat dissipation techniques are taxed. The limited air volume in the interior of a CT scanner limits the effectiveness of the rotating oil-air heat exchanger. Space constraints prevent larger heat exchangers from being accommodated on the rotating frame.
The present invention provides a new and improved cooling system which overcomes the above-referenced problems and others.